I just learned last fall (after growing iris for several years), that a rhizome only blooms once & then its purpose is to multiply. When you divide iris, you can toss the mother rhizome, although if I really like the iris I will replant the original if it is healthy, it will make more rhizomes.

A "bit" mushy (kinda corky/rubbery) sounds like just what ngraham said - the old mother rhizomes fading away. Bacterial soft rot is not a "bit" mushy - it's full-on nasty, smelly mush.

Thanks. That's reassuring.